Nicotinamide adenine dinucleotide, like all dinucleotides, consists of two nucleosides joined by a pair of bridging phosphate groups. The nucleosides each contain a ribose ring, one with adenine attached to the first carbon atom (the 1' position) and the other with nicotinamide at this position. The nicotinamide moiety can be attached in two orientations to this anomeric carbon atom. Because of these two possible structures, the compound exists as two diastereomers. It is the β-nicotinamide diastereomer of NAD+ that is found in organisms. These nucleotides are joined together by a bridge of two phosphate groups through the 5' carbons.[1]The redox reactions of nicotinamide adenine dinucleotide.

In metabolism, the compound accepts or donates electrons in redox reactions.[2] Such reactions (summarized in formula below) involve the removal of two hydrogen atoms from the reactant (R), in the form of a hydride ion (H−), and a proton (H+). The proton is released into solution, while the reductant RH2 is oxidized and NAD+ reduced to NADH by transfer of the hydride to the nicotinamide ring.

RH2 + NAD+ → NADH + H+ + R;

From the hydride electron pair, one electron is transferred to the positively charged nitrogen of the nicotinamide ring of NAD+, and the second hydrogen atom transferred to the C4 carbon atom opposite this nitrogen. The midpoint potential of the NAD+/NADH redox pair is −0.32 volts, which makes NADH a strong reducing agent.[3] The reaction is easily reversible, when NADH reduces another molecule and is re-oxidized to NAD+. This means the coenzyme can continuously cycle between the NAD+ and NADH forms without being consumed.[1]

In appearance, all forms of this coenzyme are white amorphous powders that are hygroscopic and highly water-soluble.[4] The solids are stable if stored dry and in the dark. Solutions of NAD+ are colorless and stable for about a week at 4 °C and neutral pH, but decompose rapidly in acids or alkalis. Upon decomposition, they form products that are enzyme inhibitors.[5]

UV absorption spectra of NAD+ and NADH.

Both NAD+ and NADH strongly absorb ultraviolet light because of the adenine. For example, peak absorption of NAD+ is at a wavelength of 259 nanometers (nm), with an extinction coefficient of 16,900 M−1cm−1. NADH also absorbs at higher wavelengths, with a second peak in UV absorption at 339 nm with an extinction coefficient of 6,220 M−1cm−1.[6] This difference in the ultraviolet absorption spectra between the oxidized and reduced forms of the coenzymes at higher wavelengths makes it simple to measure the conversion of one to another in enzyme assays – by measuring the amount of UV absorption at 340 nm using a spectrophotometer.[6]

NAD+ and NADH also differ in their fluorescence. NADH in solution has an emission peak at 460 nm and a fluorescence lifetime of 0.4 nanoseconds, while the oxidized form of the coenzyme does not fluoresce.[7] The properties of the fluorescence signal changes when NADH binds to proteins, so these changes can be used to measure dissociation constants, which are useful in the study of enzyme kinetics.[7][8] These changes in fluorescence are also used to measure changes in the redox state of living cells, through fluorescence microscopy.[9]Concentration and state in cells

In rat liver, the total amount of NAD+ and NADH is approximately 1 μmole per gram of wet weight, about 10 times the concentration of NADP+ and NADPH in the same cells.[10] The actual concentration of NAD+ in cell cytosol is harder to measure, with recent estimates in animal cells ranging around 0.3 mM,[11][12] and approximately 1.0 to 2.0 mM in yeast.[13] However, more than 80% of NADH fluorescence in mitochondria is from bound form, so the concentration in solution is much lower.[14]

Data for other compartments in the cell are limited, although in the mitochondrion the concentration of NAD+ is similar to that in the cytosol.[12] This NAD+ is carried into the mitochondrion by a specific membrane transport protein, since the coenzyme cannot diffuse across membranes.[15]

The balance between the oxidized and reduced forms of nicotinamide adenine dinucleotide is called the NAD+/NADH ratio. This ratio is an important component of what is called the redox state of a cell, a measurement that reflects both the metabolic activities and the health of cells.[16] The effects of the NAD+/NADH ratio are complex, controlling the activity of several key enzymes, including glyceraldehyde 3-phosphate dehydrogenase and pyruvate dehydrogenase. In healthy mammalian tissues, estimates of the ratio between free NAD+ and NADH in the cytoplasm typically lie around 700; the ratio is thus favourable for oxidative reactions.[17][18] The ratio of total NAD+/NADH is much lower, with estimates ranging from 3–10 in mammals.[19] In contrast, the NADP+/NADPH ratio is normally about 0.005, so NADPH is the dominant form of this coenzyme.[20] These different ratios are key to the different metabolic roles of NADH and NADPH.Biosynthesis

NAD+ is synthesized through two metabolic pathways. It is produced either in a de novo pathway from amino acids or in salvage pathways by recycling preformed components such as nicotinamide back to NAD+.De novo production

Some metabolic pathways that synthesize and consume NAD+ in vertebrates. The abbreviations are defined in the text.

Most organisms synthesize NAD+ from simple components.[2] The specific set of reactions differs among organisms, but a common feature is the generation of quinolinic acid (QA) from an amino acid—either tryptophan (Trp) in animals and some bacteria, or aspartic acid (Asp) in some bacteria and plants.[21][22] The quinolinic acid is converted to nicotinic acid mononucleotide (NaMN) by transfer of a phosphoribose moiety. An adenylate moiety is then transferred to form nicotinic acid adenine dinucleotide (NaAD). Finally, the nicotinic acid moiety in NaAD is amidated to a nicotinamide (Nam) moiety, forming nicotinamide adenine dinucleotide.[2]

In a further step, some NAD+ is converted into NADP+ by NAD+ kinase, which phosphorylates NAD+.[23] In most organisms, this enzyme uses ATP as the source of the phosphate group, although several bacteria such as Mycobacterium tuberculosis and a hyperthermophilic archaeon Pyrococcus horikoshii, use inorganic polyphosphate as an alternative phosphoryl donor.[24][25]

Extracellular actions of NAD+

In recent years, NAD+ has also been recognized as an extracellular signaling molecule involved in cell-to-cell communication.[36][68][69] NAD+ is released from neurons in blood vessels,[35] urinary bladder,[35][70] large intestine,[71][72] from neurosecretory cells,[73] and from brain synaptosomes,[74] and is proposed to be a novel neurotransmitter that transmits information from nerves to effector cells in smooth muscle organs.[71][72] Further studies are needed to determine the underlying mechanisms of its extracellular actions and their importance for human health and diseases.Research

The enzymes that make and use NAD+ and NADH are important in both pharmacology and the research into future treatments for disease.[75] Drug design and drug development exploits NAD+ in three ways: as a direct target of drugs, by designing enzyme inhibitors or activators based on its structure that change the activity of NAD-dependent enzymes, and by trying to inhibit NAD+ biosynthesis.[76]

It has been studied for its potential use in the therapy of neurodegenerative diseases such as Alzheimer's and Parkinson disease.[2] A placebo-controlled clinical trial in people with Parkinson's failed to show any effect.[77]

NAD+ is also a direct target of the drug isoniazid, which is used in the treatment of tuberculosis, an infection caused by Mycobacterium tuberculosis. Isoniazid is a prodrug and once it has entered the bacteria, it is activated by a peroxidase enzyme, which oxidizes the compound into a free radical form.[78] This radical then reacts with NADH, to produce adducts that are very potent inhibitors of the enzymes enoyl-acyl carrier protein reductase,[79] and dihydrofolate reductase.[80]

Since a large number of oxidoreductases use NAD+ and NADH as substrates, and bind them using a highly conserved structural motif, the idea that inhibitors based on NAD+ could be specific to one enzyme is surprising.[81] However, this can be possible: for example, inhibitors based on the compounds mycophenolic acid and tiazofurin inhibit IMP dehydrogenase at the NAD+ binding site. Because of the importance of this enzyme in purine metabolism, these compounds may be useful as anti-cancer, anti-viral, or immunosuppressive drugs.[81][82] Other drugs are not enzyme inhibitors, but instead activate enzymes involved in NAD+ metabolism. Sirtuins are a particularly interesting target for such drugs, since activation of these NAD-dependent deacetylases extends lifespan in some animal models.[83] Compounds such as resveratrol increase the activity of these enzymes, which may be important in their ability to delay aging in both vertebrate,[84] and invertebrate model organisms.[85][86] In one experiment, mice given NAD for one week had improved nuclear-mitochrondrial communication.[87]

Because of the differences in the metabolic pathways of NAD+ biosynthesis between organisms, such as between bacteria and humans, this area of metabolism is a promising area for the development of new antibiotics.[88][89] For example, the enzyme nicotinamidase, which converts nicotinamide to nicotinic acid, is a target for drug design, as this enzyme is absent in humans but present in yeast and bacteria.[30]

In bacteriology, NAD, sometimes referred to factor V, is used a supplement to culture media for some fastidious bacteria.[90]